Poly(alkylene glycol aromatic acid esters) have found use in molding resins because of their rigidity, good dimensional stability, low water absorption and good electrical properties. These resins also produce molded articles having high heat resistance, inherent lubricity and excellent chemical resistance. One restriction on the use of these valuable resins, however, is the fact that the impact strengths of moldings tend to be somewhat inadequate for applications where the molded part is likely to be subjected to severe service conditions. This has led to work to upgrade these properties of poly(alkylene glycol aromatic acid esters) because, both in straight, branched and cyclic alkylene chain modifications, they are so superior to many other molding materials, especially with respect to surface gloss when molded.
It has now been discovered that if poly(alkylene glycol aromatic acid ester) resins are chemically modified by being segmented in a copolyester in which the major portion of the repeating units are (a) poly(alkylene glycol aromatic acid ester) units and the minor portion of the repeating units are units of (b) a di-ester comprising an aromatic diol, then the resulting copolyesters will have enhanced impact resistance, compared to the resin (a) itself. The improvement in impact resistance is achieved with minimal loss of other physical properties and is accompanied with a measurable increase in toughness. It is believed that the presence of the internal units of other aromatic di-esters modifies the rate at which such poly(alkylene glycol aromatic ester) resins crystallize from the melt in a very desirable manner.
Illustratively, if certain aromatic diol-containing di-esters are added to the reactor during the preparation of poly(1,4-butylene terephthalate) by ester interchange between dimethyl terephthalate and 1,4-butanediol, either as part of the initial charge, or after the prepolymerization stage is complete, or if they are reacted with a terminally-reactive poly(1,4-butylene terephthalate) resin, there is caused a most desirable modification in the properties of the resulting, modified polyester molding resins.
By way of illustration, succinic anhydride and maleic anhydride are each reacted, respectively, with bisphenol-A to produce carboxyl-terminated di-esters, and these are used as the source of units. These carboxyl-terminated di-esters are added, respectively, to a reactor with 1,4-butanediol and dimethyl terephthalate and dibutyl tin dilaurate and heated under moderate, then high vacuum until the formation of copolyester is substantially complete. Alternatively, 1,4-butanediol and dimethyl terephthalate can be heated to form .beta.-hydroxyethyl terephthalate, the di-ester added, and the copolyester is formed on further heating. In still another procedure, the di-ester derived from the aromatic diol is reacted with high molecular weight poly(1,4-butylene terephthalate) having functionally reactive end groups, and a small amount of 1,4-butanediol and copolymerization occurs.
No matter how they are made, however, after completion of the reaction and molding the copolyesters, the articles obtained are improved in toughness and reduced in notch sensitivity as compared to bars molded from the corresponding unmodified poly(alkylene aromatic acid esters). There is insubstantial loss in flex modulus and strength.
The effect on crystallization behavior is also noteworthy. The aromatic diol-derived di-ester components significantly reduce the crystallization rate of the molding resin. This is desirable, since it allows longer time for the polymer melt to flow through thin walled sections of a mold before the cooling product solidifies.
In addition to their use in injection molding application, the di-ester coreactants have also been found to be beneficial in improving the properties of poly(alkylene glycol aromatic acid ester) resins used in other applications, such as profile extrusion, extrusion-and injection blow molding, thermoforming, foam molding; in these cases small amounts of ester-forming branching agents may be added to enhance the melt elasticity properties of the products for easier processing.
The copolyester products can also be converted to valuable modifications by adding reinforcing fillers, such as glass fibers, talc, and the like, and/or flame retardant agents such as monomolecular or polymeric halogenated aromatic compounds, with and without flame retardant synergists, such as antimony compounds, or phosphorus compounds, and the like. Surprisingly, the increased toughness of the new copolyesters compensates for the greater brittleness usually induced by the incorporation of such non-soluble additives and fillers.